glaeken wrote:
I heard the origin is from the Middle Ages when apparently people thought you could be possessed by a demon when you sneezed. The bless you comment some how saves you from possession.

No idea if any of that is true.

I think you're correct. It was thought that when you sneezed, you were most vulnerable to attack from spirits or demonic possession. Also it was thought that sneezing was releasing evil spirits from your body and blessing the person prevented them from re-entering the person.

Saying bless you after someone sneezes is, as people have pointed out, because sneezing was the first outward sign of the plague, and they were trying to get God to help you out.

The one about being possessed by the devil is having to put your hand over your mouth when you yawn. Because you can't stop opening your mouth when you yawn, people thought the (invisible) devil was forcing it open to get inside for a bit of possessing. Covering you mouth would stop him. In the middle ages that is.